The invention relates to cooling and purge air-flow systems for gas turbines.
A gas turbine engine conventionally includes a compressor for compressing ambient air for being mixed with fuel and ignited to generate combustion gases in a combustor. A turbine receives the hot combustion gases and extracts energy therefrom for powering the combustor and producing output power, for example for powering an electrical generator. The turbine conventionally includes one or more stages of stator nozzles or vanes, rotor blades and annular shrouds around the turbine blades for maintaining appropriate clearances therewith. As turbine inlet temperatures have increased to improve the efficiency of gas turbine engines, it has become necessary to provide a cooling fluid such as air to the turbine vanes, blades and shrouds to maintain the temperatures of those components at levels that can be withstood by the materials thereof, to ensure a satisfactory useful life of the components. Cooling is typically accomplished by extracting a portion of the air compressed by the compressor from the compressor and conducting it to the components of the turbine to cool the same. Any air compressed in the compressor and not used in generating combustion gases necessarily reduces the efficiency of the engine. Therefore, it is desirable to minimize the amount of cooling air bled from the compressor. Furthermore, the air used for cooling turbine components typically discharges from orifices or gaps in those components. That cooling air mixes with the combustion gases in the turbine and will also reduce engine efficiency for thermodynamic and aerodynamic reasons. Accordingly, while turbine efficiency increases as turbine inlet temperature increases, that increase in temperature also requires effective cooling of the heated components, and such cooling is optimally effected in a manner so as not to forfeit the increased efficiency realized by the increased temperature.
The axial location or stage where the air is bled from the compressor is determined by the pressure required by the component or system to be serviced by that air. To ensure sufficiently high delivery pressure, in general it is desirable to select the source with the highest possible pressure. However, bleeding air from the earliest possible stage of the compressor will increase compressor efficiency by reducing the amount of work invested in the extracted air. Therefore, it is desirable to achieve the highest possible system supply pressure from the earliest and lowest pressure stage of the compressor.
Furthermore, the cooling air must be provided at suitable pressures and flow rates to not only adequately cool the turbine component(s) but to maintain acceptable back flow margin(s). Back flow margin is defined as the difference between the cooling air pressure inside, for example, the outer side wall, and the pressure of the hot combustion gases which flow through the turbine. A positive back flow margin is desirably maintained so that combustion gases are not ingested into the outer sidewall.
The pressure in the outer side wall cavities of the third and fourth stage nozzles is critical for setting back flow margin (BFM). As noted above, the purpose of BFM is to stop ingestion of hot flow-path gas into the outer side wall cavities. BFM is the percentage that the cavity pressure is above the maximum pressure in the associated gas path. Usually, for a nozzle stage, the maximum pressure will be the maximum stagnation pressure on the airfoil lead edge. Conventionally, the minimum back flow margin (e.g. 3%) is set according to the worst conditions to which the component is to be exposed. In the case of some gas turbine systems, worst conditions occur at Cold Day Turn Down (CDTD), typically about 0-degrees and half power. Under these conditions, maximal compressor bleed flow is appropriate because the compressor bleed pressure is low compared to the compressor outlet pressure. When the BFM is set at CDTD, however, the actual back flow margin will increase away from that at CDTD because under other conditions, the bleed pressure from the compressor is higher than at CDTD. If the BFM is too large, the outer side cavities will leak excessively and as a result combined cycle performance will deteriorate. In that regard, as noted above, a negative consequence of this excessive leakage is a performance loss because the work/power used to compress that air is wasted.
To balance these two opposing requirements, the invention is embodied in a cooling and purge air supply system wherein the pressures in the outer side wall cavities are modulated, based on compressor discharge pressure (Pcd) in the presently preferred embodiment, thereby to generally maintain the BFM so as to minimize excessive leakage and the consequent performance deterioration.
Thus, the invention may be embodied in a cooling air flow control system for a gas turbine. In such an embodiment, the cooling air flow control system comprises a valve that controls the pressure of cooling air conducted from the compressor to the turbine and a controller for controlling the valve in accordance with an operating condition of the gas turbine, for example, the compressor discharge pressure. In an exemplary embodiment, during at least a portion of the operating range of the gas turbine, the control system controls the valve to substantially maintain the air pressure within a component of the turbine at a value that is a prescribed percentage of the concurrent compressor discharge pressure.
The invention may also be embodied in a system and method for providing cooling and/or purge air from a multi-stage compressor to an associated turbine for cooling at least one component of the turbine and/or preventing back flow of combustion gases thereinto wherein the pressure of the cooling and/or purge air is controlled according to an operating condition of the turbine, for example, the compressor discharge pressure. In an exemplary embodiment, the pressure of the cooling and/or purge air is controlled to substantially maintain the air pressure within the component of the turbine at a value that is a prescribed percentage of the concurrent compressor discharge pressure.